This invention relates to a shadow mask for use in a color CRT (cathode ray tube).
A shadow mask for use in a color CRT is arranged in proximity to a tri-color fluorescent screen. Electron beams emitted from electron guns pass throug a corresponding electron-beam hole in a shadow mask and land precisely on predetermined spots on the fluorescent screen. The shadow mask functions to permit the electron beam to be directed onto the fluorescent screen. For this reason, the relative position, diameter, and configuration of the electron-beam holes directly influence the resultant image quality, and a high manufacturing accuracy, therefore, is required when forming the electron-beam holes. Low manufacturing accuracy would lead to the degeneration of image quality, resulting from a "doming" phenomenon. In order to prevent the generating of scattering electrons, use of a special process is required which makes the beam-exit surface side of each electron beam hole larger than its beam-entry surface side, and thus makes the electron-beam hole semispherical in configuration.
Conventionally, high manufacturing accuracy of the electron beam holes has been achieved by etching a shadow mask plate with a predetermined etchant, to form electron-beam holes.
Recently, due to widespread demand for television pictures having "fineness of texture", a high quality television system is being developed as a communication system. The high quality television system requires a CRT having improved image resolution. In order to improve image resolution, it is necessary to form the electron-beam holes of a shadow mask more finely, and achieve a high density thereof.
In order to provide very fine, high-density electron-beam holes in the shadow mask, it is required that they be formed with higher accuracy than has been required in the case of the conventional CRT. However, the electron beams tend also to strike the area surrounding the individual beam holes, which causes the shadow mask to become heated and to expand, with the result that the electron-beam holes become somewhat displaced from their proper positions to cause a new problem of occurrence of a color deviation phenomenon called "purity drift" (hereinafter referred to as "PD"). An invar type alloy of a low thermal expansion coefficient has been used for the shadow mask so as to lower the PD value. In this case, it has, however, proved difficult to form electron-beam holes by use of a conventional etching method, because the invar type alloy has a high resistance to etching. Consequently, even if it is possible to form electron-beam holes in this alloy, it is difficult to form them with the required high accuracy.
In connection with this problem, the inventors demonstrated, in U.S. Pat. No. 4,528,246, the concept of forming fine electron-beam holes on a shadow mask plate with better accuracy by forming {100} crystal faces, by rolling and heating, the mask plate surface to have the {100} texture of 35% or more. Here, an f value has been adopted as a parameter the {100} texture on the surface of the shadow mask. However, it could be that the accuracy with which the electron-beam holes are made may be inadequate, thus making it difficult to evaluate such an "f" value, the parameter of the {100} texture.